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Some dietary advice for athletes at the London Olympics: Watch how much liver you eat. Too much can raise the risk of testing positive for the banned substance clenbuterol.

The official advice from Britain comes after the World Anti-Doping Agency issued a warning last month about the dangers of athletes eating contaminated meat.

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Clenbuterol is on WADA's list of banned substances as an anabolic agent that builds muscle and burns fat, and athletes who test positive can face bans of up to two years.

In advice based on evidence from the Food Standards Agency, U.K. Anti-Doping warns: "Athletes should be wary of consuming large quantities of liver."

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Spanish cyclist Alberto Contador blamed contaminated steak from Spain after he tested positive for clenbuterol during his 2010 Tour de France victory. His case is under appeal with the Court of Arbitration for Sport.

Contaminated meat was also blamed when five Mexican soccer players tested positive for clenbuterol before the Gold Cup in June. FIFA said more than 100 players at the Under-17 World Cup in Mexico also were contaminated by the substance.

The British risk assessment is based on eating bovine liver, one of the organs where the highest concentration of clenbuterol lingers.

"The FSA cannot rule out the possibility that if a large portion of liver is consumed containing clenbuterol at permitted residue limits, urine collected shortly after consumption may contain detectable levels of clenbuterol," UKAD said Thursday to The Associated Press. "This depends on many factors including the amount consumed, the timing of the urine test and the analytical methods used."

UKAD was not able to quantify the amount of liver that could put athletes in danger of failing a test.

As far back as 1999, Chinese swimmers Wang Wei and Xiong Guoming blamed a dish of stir-fried pig liver for their out-of-competition clenbuterol positives.

Athletes have successfully had doping bans overturned by arguing they had competed in countries where many farmers feed clenbuterol to livestock.

But clenbuterol itself is prohibited in the European Union for growth promotion in food producing animals.

UKAD said it has been "reassured that the European Union has stringent rules in place to prevent contamination" and that meat can only be imported into the region if "those countries can offer guarantees that the produce meets EU standards."

WADA has urged athletes to exercise "extreme caution" when eating meat in China and Mexico, but both those countries are not able to export meat into the 27-nation EU bloc, which includes Britain.

"It is, however, authorized for some therapeutic uses, such as treating respiratory diseases in horses and as a muscle relaxant in pregnant cattle, usually during calf delivery," UKAD said. "Authorized veterinary medicinal products containing clenbuterol can only be prescribed by a veterinarian.

UKA said veterinary experts in the U.K. carry out searches for residues of clenbuterol and have found "no evidence of illegal use."